It may be necessary to measure a current within a power converter and/or control a power converter utilizing a measurement of a current which is inaccessible. As used herein, the term “inaccessible” includes, but is not limited to, a situation in which the power converter cannot access a measurement of a current within a load, because they are physically separate, for example. The load may not have means for measuring the desired current and it may not be possible to add such means or provide access to such measurement for a physically separate power converter. The term “inaccessible” is also intended to mean the situation in which the measurement of the current may be accessible, but the measurement may be difficult and/or expensive to make. For example, the result of the measurement, which may be a voltage, may be referred to a different ground or to a higher voltage, thus making a measuring or control circuit both difficult and/or expensive to make, in addition to being complex. Another possibility is that the current to be measured may be on the wrong side of the isolation barrier where it is undesirable to penetrate the barrier. Furthermore, the term “inaccessible” is intended to cover the situation in which the measurement of the current may be accessible and the cost and/or complexity of the circuit needed to make the measurement may be reasonable, but the measurement requires a high power dissipation. This can occur, for example, if a resistive shunt is used to measure the current in a high current power converter where producing a usable voltage across the shunt (e.g. 100 mV) may result in high power dissipation due to the high current passing through the shunt. Those skilled in the art will recognize that there are other situations in which the current desired to be measured and/or used to control a power converter may be “inaccessible”.
It should be noted that although power converter circuits are sometimes called “regulators,” the term “power converters” or “converters” as used herein should be considered as referring to a buck circuit, a boost circuit, or a buck-boost (flyback) circuit.
Thanks to the work of Vorperian and others who proposed a pulse width modulation (PWM) switch concept, there are known relationships between different average currents in the three types of converters, if the converter is operating in transition mode (TM) or continuous current mode (CCM). These relationships are:Buck: Isw=Iin=Iout*D=IDiode*D/(1−D)  equation (1)Boost: Isw=Iin*D=Iout*(1−D)/D=IDiode*(1−D)/D  equation (2)Buck-Boost: Isw=Iin=Iout*(1−D)/D=IDiode*D/(1−D)  equation (3)
where ISW is the current in the switch;                IIN is the input current to the converter;        IOUT is the output current from the converter;        IDiode is the current in the rectifier diode or synchronous switch;        D is the duty cycle of the PWM switch; and        (1−D) is the duty cycle of the rectifier.        
As can readily be seen by analysis of the equations, if the desired current to be measured or controlled is “inaccessible” (as defined herein), we can sense a more convenient current and then solve the appropriate equation from equations 1-3 to obtain a replica of the desired current which can then be used to provide the measurement and/or can be used as the control variable.
A problem with solving one of these equations is that it requires multiplication and/or division. As is well known to those skilled in the art, analog multiplication and division circuits are inaccurate, expensive, take up a lot of room on an integrated circuit and consume considerable power. On the other hand, utilizing digital circuits for this purpose requires an ADC circuit at the input and a DAC circuit at the output, if analog control is to be implemented. Thus, digital multiplication or division does not provide the desired solution, either.
Thus, there is a need for a simple, inexpensive, high-performance and reliable circuit to permit analog multiplication or division within a power converter to enable the use of inaccessible current measurement to provide a desired current for measurement and/or control in the power converter.